1. Field of the Invention
The present invention relates to devices and methods for Global Navigation Satellite System (GNSS) receivers. More particularly, the present invention system relates to acquiring and tracking GNSS signals and to extracting time and frequency parameters thereof, as exemplified by the direct sequence spread spectrum (DS/SS) signals used by the operational U.S. NAVSTAR Global Positioning System (GPS), the Russian Global Navigation System (GLONASS) and the planned European GALILEO System, for positioning, timing, and other measurement applications.
2. Description of the Prior Art
The NAVSTAR Global Positioning System (GPS) will play an important role in the future global navigation satellite system (GNSS). Other important parts of the GNSS may include the Russian Global Navigation Satellite System (GLONASS) and the European GALILEO System currently under construction. The today fully operational GPS, which is maintained by the government of the United States of America, is also undergoing significant modernization in its space and control segments with new user equipment being developed as well.
GPS radio navigation relies upon a constellation of twenty four satellites that are in six different orbital planes around the earth. A navigation solution is obtained through measurements of propagation delay times of the radio signals broadcast by the orbiting satellites to a user. Normally, a user must receive signals from at least four satellites in order to solve for the variables of longitude, latitude, altitude and time that are needed to precisely determine location.
Each GPS satellite transmits a radio signal at several frequencies including 1575.42 MHz (referred to as L1) and 1227.60 MHz (L2) and soon 1176.45 MHz (L5). Each radio signal carries information on its in-phase and quadrature components. Each carrier signal component is phase-modulated with at least one pseudo random number (PRN) code, known as the spectrum spreading code. Some are further modulated with a navigation data message that provides the precise satellite orbital, clock, and other information. Each spectrum-spreading code is unique for a satellite and is used as the identifier for that satellite. Since GPS satellites are equipped with and controlled by a set of ultra precise atomic oscillators, the GPS signal carrier phase and the modulating codes are exactly the reading of the onboard atomic clock's GPS time at which the signal is transmitted. As a result, acquisition of a GPS satellite signal and decoding its code provides the exact knowledge of the GPS satellite orbital position and the time at which the received signal was transmitted.
When traveling from a GPS satellite to a near earth surface user, the GPS signal experiences a propagation delay about 76 ms. The length of the propagation delay directly relates to the distance between the GPS satellite at transmission and the user at reception and is affected by the presence of ionosphere and troposphere and some weather conditions. Due to the relative motion between the transmitting satellite and the user, the GPS signal is either stretched or squeezed at reception, as compared to its original form at transmission. This is the so-called Doppler effect, and the change incurred to the signal is the Doppler frequency shift.
A GPS receiver is used to measure the propagation delay (range) and Doppler frequency shift (range rate) of each received signal and to demodulate the navigation data. Since the transmission time is embedded in the signal carrier and code phase, when a locally generated copy of the signal carrier with code (called the reference or replica) is so adjusted to match up with the incoming signal in both time and frequency and phase, it duplicates the satellite clock reading at transmission (a transmission time tag). Each GPS receiver has its own clock, albeit inaccurate, and uses it to mark the signal at reception relative to this local time base (a reception time tag). The difference between the two time tags is a measurement of the signal propagation delay and the range to each satellite is then calculated by multiplying each delay by the speed of light subject to some corrections.
A successful correlation between the reference code and the incoming signal identifies which GPS satellite signal is being received. This also removes the spectrum-spreading code and this despreading process increases the signal to noise ratio (SNR) and allows the only-remaining navigation data bits to be demodulated. The navigation data bits provide the precise orbital location of the satellite and coefficients for error-correcting formulas. The location and time of the user are then found by solving known equations that incorporate the measured range to the known location of the GPS satellites.
A typical GPS receiver consists of four subsystems. Those subsystems include a radio frequency (RF) front-end with a low noise-amplified antenna, a baseband signal processor, a navigation data processor, and a user interface. The RF front-end down converts the GPS signal from the RF at a GHz range to an intermediate frequency (IF) at a MHz range suitable for sampling and quantization. A conventional baseband signal processor has twelve or more identical tracking channels, each assigned to a GPS satellite in view and to new satellites emerging above the horizon. Each tracking channel is made of a code tracking loop and a carrier phase or frequency tracking loop. Code and carrier tracking loop in turn contains mixers, accumulate and dump (i.e., early, prompt, late correlators for both in-phase and quadrature components), code delay and carrier phase/frequency error discriminators, loop filters, code and carrier numerical controlled oscillators (NCOs), and code and carrier generators, respectively. Code phase and carrier phase/frequency measurements are taken from the tracking channels, from which pseudorange, delta range, and integrated beat carrier phase observables are generated. In addition to code and carrier tracking loops closure at a kHz range, the baseband signal processor also conducts navigation data demodulation and GPS observable generation at appropriate rates. Finally, the navigation data processor operates on raw observables with a Kalman filter or a least-squares estimator at a several Hz range for GPS satellite orbit calculation, navigation solution, and data input/output to the user.
Since the inception, GPS receiver technologies have steadily progressed and made transitions from analog to digital, from single-channel sequential-multiplexing to multiple-channel parallelism, and from GPS-alone to an integrated GNSS receiver. These prior art GPS receivers share a common architecture composed of two separate local loops. In the downstream is a navigation loop and in the upstream are a delay-locked loop (DLL) and a phase/frequency-locked loop (PLL/FLL), which are closed by software around correlators.
At the heart of prior art GPS receivers are the correlators implemented either in hardware or software which produce the correlation (or the mainlobe of it) of the underlying PRN spreading codes. Clearly the shape of the correlation function depends on the particular PRN codes used. For example, the binary phase shift keying (BPSK) modulation code has a single correlation peak whereas the binary offset carrier (BOC) modulation code has a mainlobe and several sidelobes that do not die down quickly. For the BPSK type of codes, the ideal correlation function is a triangle with its base being a code chip wide on either side (e.g., ±300 m for the GPS C/A-code and ±30 m for P-code). Any multipath signals with an additional delay shorter than one code chip will interfere with the correlation of the direct signal, distorting its correlation function shape and leading to ranging errors. For the BOC type of codes such as the GPS M-code, on the other hand, extra special hardware and software have to be used to deal with sidelobes to avoid missing detection, false acquisition, and lock onto a sidelobe with biased measurements.
A need therefore exits for a novel GNSS receiver architecture and its enabling signal processing techniques that are less sensitive to multipath and do not require extra hardware and/or software for operation under different spreading codes. This need is met by the present invention as described and claimed below.